The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) standards propose using an Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. In an OFDMA communication system, a frequency channel, or bandwidth, is split into multiple channel elements during a given time period. In the control channel, each control channel element comprises 36 orthogonal frequency sub-carriers over a given number of OFDM symbols, which are the physical layer channels over which channels are transmitted in a TDM or TDM/FDM fashion. In the physical domain, the control channel elements are distributed throughout the bandwidth. A control channel then comprises one or more channel elements (that is, control channel elements (CCEs)) that are distributed across the entire bandwidth, which CCEs are logically contiguous but physically diverse. That is, control signaling, such as downlink (DL) and uplink (UL) grants and power control signaling, are transmitted using a Physical Downlink Control Channel (PDCCH) that, in turn, consists of 1, 2, 4, or 8 logically contiguous, but physically diverse, CCEs.
Typically, the number of CCEs allocated to the PDCCH for a given user/user equipment (UE) is determined by reference to a lookup table and based on Channel Quality Information (CQI) reported by the UE. The worse the reported channel conditions, the greater the number of CCEs allocated to the UE. For example, when a CQI value of 15, corresponding to good channel conditions, is reported by a UE then a single CCE may be allocated to the UE for a DL and/or an UL grant, and when a CQI value of 2, corresponding to poor channel conditions, is reported by a UE then eight CCEs may be allocated to the UE for a DL and/or an UL grant.
In addition, the 3GPP LTE standards provide for limiting the range of CCEs that may be allocated to a particular UE for a PDCCH in order to minimize a search by a UE for its allocated PDCCH. That is, a UE does not know in advance where, among all possible CCEs of a sub-frame, to find its allocated PDCCH. In order to limit a maximum number of blind decodes performed by a UE in order to determine whether a PDCCH has been allocated to the UE, common and UE-specific CCE search spaces are pre-defined by an algorithm maintained by each UE.
For example and referring now to FIG. 1, a block diagram 100 is provided that illustrates an exemplary allocation of search spaces and logical CCEs of a sub-frame to users' equipment (UEs) served by an eNodeB in accordance with the prior art. A first group of logical CCEs 0-15 are a common search space in which any UE served by the eNodeB may be allocated a PDCCH, with the result that all UEs served by the eNodeB will search this space for their PDCCH and corresponding DL/UL grant. A second group of CCEs 16-43 are a UE-specific search space in which only a subset of all UEs served by the eNode B, for example, UEs 101-104, may be allocated a PDCCH, with the result that only that subset of UEs will search this space for their PDCCH and corresponding DL/UL grant. As a result, all UEs served by the eNodeB will search CCEs 0-15 to determine whether they have been allocated a PDCCH and granted a DL or UL, whereas only the subset of UEs, that is, UEs 101-104, will also search CCEs 16-43 to determine whether they have been allocated a PDCCH and granted a DL or UL.
Due to the use of the UE-specific search spaces, some users/UEs may be blocked from allocation of a CCE, and correspondingly cannot be scheduled for a DL or UL transmission even though CCEs are available. For example and again referring to FIG. 1, based on CQI feedback, the serving eNodeB has granted a DL and/or UL channel to each of UEs 101 and 104 and has allocated four consecutive logical CCEs, that is, CCEs 16-19 and 23-26, to a control channel, that is, a PDCCH, for transmission of the grants to UEs 101 and 104, respectively. The serving eNodeB also has granted a DL and/or UL channel to UEs 103 and has allocated two consecutive logical CCEs 42-43 to a PDCCH for transmission of the grant to UE 103. Typically, UE 103 is allocated fewer CCEs than UEs 101 and 104 because channel conditions are better between the eNodeB and UE 103 than between the eNodeB and UEs 101 and 104, and correspondingly the control channel between the eNodeB and UE 103 will utilize a higher level modulation scheme, a lower coding rate, and/or a lower bit repetition rate than the control channels between the eNodeB and UEs 10 and 104, and therefore needs fewer CCEs. However, UE 102 is blocked from a CCE allocation and therefore cannot be scheduled for a control channel transmission, and correspondingly cannot be granted a DL or UL channel, even though logical CCEs 20-22 and 27 are available.
The blocking of UEs from being scheduled for a control channel, with the result that those UEs cannot be granted a DL or UL channel, even though logical CCEs are available for assignment to those UEs results in a longer latency for data transfers to and from the blocked UEs, a lower system throughput, and a waste of system capacity. As a result, a need exists for an improved method and apparatus for scheduling a control channel for a UE in a 3GPP LTE communication system.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.